chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/pmtud/ip/source.go

@@ -29,7 +29,7 @@ func SrcAddr(dst netip.AddrPort, proto int) (src netip.AddrPort, cleanup func(),
 }
 
 var (
-	errNoRoute            = fmt.Errorf("no route to destination")
+	errNoRoute            = errors.New("no route to destination")
 	ErrNetworkUnreachable = errors.New("network unreachable")
 )